In modern telephone networks, the voice path between a calling party and a called party and the signalling path which is used to control call-processing (call setup, teardown, billing, etc.) are distinct. This network architecture is known as a “Common Channel Signaling” (“CCS”) architecture and is common throughout the North American Public Switched Telephone Network (“PSTN”) and other telephone carrier networks worldwide.
The CCS architecture enables rapid call setup and teardown as well as many advanced services (i.e. Alternate Billing Services, “800” toll-free calling, Intelligent Network services, wireless roaming, Local Number Portability). In the CCS, telephone switches are connected directly to each other by voice trunks (as in previous architectures) and are also connected by a parallel network of signaling links, which are point-to-point digital data circuits generally operating at 56 kilobits per second or 64 kilobits per second. When a telephone call is initiated, messages are sent across the signaling links to cause voice trunks to be allocated from switch to switch in the call path. Throughout the duration of the call, and at the end of the call, other messages are sent back and forth across the signaling links, independently of the voice path established between calling and called party over the separate, parallel network of voice trunks.
Generally, in the CCS architecture, voice switches (“SSP”, “Service Switching Points” or “Signal Switching Points”) are connected by signaling links to specialized packet switches or “Signal Transfer Points” (“STP”s) which route signaling messages among themselves and to the SSPs which may be their originating or destination points. Advanced services are provided by other nodes in the network such as “Service Control Points” (“SCP”s), databases which store call procedures and which are connected to STPs by signaling links. For example, when an “800” toll-free call is dialed, the caller's SSP sends a query message across a signaling link to an STP, which routes it across one or more further signaling links (possibly transiting other STPs) to an SCP, which responds by sending a message containing the actual telephone number to which to route the call, across the signaling network to the SSP, which then sends further signaling messages across the signaling network to other SSPs in order to set up voice trunks and connect the call. A single call routed across voice trunks may actually involve many distinct exchanges of signaling messages across the signaling links in the signaling network, though to calling and called party the existence of the signaling network is never evident at all.
Obviously, were nodes in the CCS network connected by single signaling links from point to point, a link failure might have grave consequences. With no means of transmitting or receiving the signaling messages which control voice trunks, switches could not set up or tear down calls; whether the voice trunks themselves were in or out of service, the switches could not even attempt to place calls across them, and many or most calls from switches isolated from the CCS network by signaling link failure would simply never be completed. Other related failures would impact every service provided by such an isolated switch which made use of the CCS network.
To prevent such catastrophic failures, links in the CCS network are grouped together into “linksets”; linksets are sets of signaling links connecting two nodes in the network which are both redundant, meaning that there is more than one link from point to point, and physically diverse, meaning that the redundant links are never on the same physical facilities at any point between the two ends of the circuit (i.e. two links which are “physically diverse” must never be multiplexed onto the same cable, must never be routed through devices which draw electrical power from the same circuits, must not be placed on two separate cables which travel in the same underground conduit, nor in any other way be subject to a single physical event which might eliminate both links between the two endpoints). Generally, networks are engineered for at least three-way redundancy and physical diversity. This involves great expense and effort; and as the underlying physical facilities change (i.e. two separate fiber optic cables are replaced by a single cable with a higher capacity) great care must be taken to ensure that redundancy and physical diversity are maintained.
Clearly, a means of automatically providing this redundancy and diversity would save great expense and effort, and potentially improve the reliability of the PSTN. Furthermore, as the expense and effort of provisioning and maintaining redundant and physically diverse signaling links is a major factor preventing many smaller carriers (such as small rural telephone companies, competitive local exchange carriers (“CLEC”s), etc.) from adopting the CCS architecture and using it (instead of older methods) to connect to the PSTN, a means of automatically providing this redundancy and diversity would be of great utility to such carriers.
Where other contemporary network architectures deploy many relatively inexpensive, high-performance packet routers, the CCS signaling network generally deploys a small number of extremely large and complex routers each aggregating tens or hundreds of signaling links. A consequence of this architectural decision is that these routers (STPs) are extremely expensive. While the router connecting a local area network to the backbone of a Transmission Control protocol/Internet Protocol (“TCP/IP”) network may have four network interfaces (or even less) and cost under $5,000, it is extremely unusual to find a STP with less than dozens of signaling links, and STP prices are at least ten times those of small or midTCP/IP routers. At the same time, STPs offer comparatively little performance to justify their extremely high prices. While even the most inexpensive TCP/IP routers can typically route a message while imposing a delay of less than one millisecond, contemporary overviews of STP architecture and performance discuss message-routing times of ten milliseconds or less as normal.
Efforts have been made to improve upon call handling systems, for example as set forth in U.S. Pat. No. 5,737,404 to Segal. Other systems include those depicted in PCT publications WO 9744962 and WO 9738537.
Clearly, it would be desirable to reduce the cost of STPs while improving their performance. Those are objects of the present invention.